Occultation determination of Neptune's oblateness and stratospheric methane mixing ratio

The occultation of a star by Neptune on 20 August 1985 was observed at 2.2 μm wavelength with telescopes at the European Southern Observatory (ESO) and the Cerro Tololo Inter-American Observatory (CTIO). The detection of a 'central flash' midway between immersion and emersion has allowed the determination of Neptune's oblateness, ε, and the atmospheric extinction at 2.2 μm, which is related to the stratospheric methane mixing ratio. We find ε = (2.08^+0.19_-0.18) × 10^-2 and, assuming a stratospheric temperature of 120 K, infer a value of 0.6% (with an uncertainty of a factor of 10) for the methane mixing ratio (CH₄/H₂) at 0.3 mbar. The latter value may indicate supersaturation of methane in Neptune's stratosphere.